Turning: the problems of bar machining/2
WE RESUME OUR ANALYSIS OF SINGLE-SPINDLE, MULTI-SPINDLE AND CNC MACHINES WORKING WITH BAR.
We resume our analysis of single-spindle, multi-spindle and CNC machines working with bar. It is well known (see article link) that machining from bar is the most common production system, but this also has its problems, which we will analyse after looking at vibration problems, inertia instability and the various effects on the machine.
Effects on cycle time › To hide the effects of induced vibrations, an expedient is often resorted to, which consists of limiting the rotation speed of the bar during machining.
"If the spindles are rotating at 6,000 rpm and there are vibrations, you can reduce the speed to, say, 4,000 rpm and there I solve the problem!" someone may think. True, but this will also have consequences for costs: by reducing the speed, you increase the cycle time. This means that I will produce less, which is even more evident when machining small bars, for which higher rotation speeds would normally be required. The smaller the bar, in fact, the lower the tool contact speed will be. In order to optimise the cutting parameters, it would therefore be necessary to increase the rotation speed of the bar, but this is prevented by the increase in vibration that would be generated. With cutting speeds that are too low, on the other hand, it will not be possible to obtain good surface finishes, with the aggravating factor that a tool working under these conditions will in turn wear out more quickly, thus increasing tooling costs.
Even in the case of larger bar sizes (60-80 mm), an increased cycle time effect must be considered, caused by the need to significantly reduce the bar's rotational speed compared to optimum cutting speeds.
In fact, the unbalancing of large bars causes even greater vibrations, given the large mass that is put into rotation; the vibrations given by an eccentric mass are in fact proportional to the mass itself and to the square of the rotation speed.
FC= m × π2 × r
FC: centrifugal force (force of inertia)
m: mass of the rotating bar
π: rotational speed
Requirement to have many types of machines of different sizes › Due to the constraint of bar passage through the spindle, the bar machine manufacturer is forced to make series of machines according to the diameter of the bar to be introduced through the spindle (bar passage). This implies that I must have machines suited to very defined ranges of workpieces: for example, if for small diameter bars (up to 25 mm) I will work on one machine, if a workpiece with a diameter of 26 mm is to be produced, I will be forced to use another machine, of a larger size. In order to be able to machine the full range of parts that are generally produced from bar stock, I will therefore need a number of different machines, each of which will be dedicated to a limited size range.
Bar feeder problem › Bar feeder systems are accessories external to lathes, used to position bars within the spindles. They can feed a single bar, or several bars, as is the case for multi-spindles. These accessories, rigidly connected to machines, introduce criticalities and can lead to problems that are worth highlighting. To begin with, each machine is equipped with a dedicated loader that is often bulkier than the machine itself. Furthermore, the fact that each loader is rigidly connected to the turning machine has the direct consequence that mechanical problems and vibrations caused by the loader are transferred to the machine, increasing the risk of inefficiency of the loader-turning system.
Waste of time › The time required to introduce new bars should be considered as dead time to be taken into account when calculating the actual cycle time.
Parting time › Yes, this must also be considered. Working from the whole bar, the parting time cannot be neglected, i.e. the time taken to cut the bar itself, which is obviously more significant for larger diameter bars.
Environmental problems › The continuous handling of bundles of bars, which are bulky (bars can be as long as 4 or 5 metres) and considerably heavy (even 2,000 kg each), especially within production departments where dozens and dozens of machines are installed, is an aspect that entails risks and safety problems to which ever-increasing attention is rightly paid.
To sum up, bar machining is an evergreen that most bar lathe users accept as a closed box, without thinking about the real implications it entails. Is it really worth the hassle and cost, however, without carefully evaluating the possible alternatives?
My advice is this: think different! or, translated into other terms, find a way to distinguish yourself. The side effect of 'you've always done it this way' is that you then fail to differentiate yourself from your competitors, and risk falling into the infamous price war in which you will ultimately be defeated. Because if you produce in the same way and with the same problems as your competitors, you will always find someone willing to offer a lower price than yours.